In the given case, Mr.X does not exceeds the threshold limit of Rs.60lakhs therfor he is not require to do tax audit. However assessee can opt special provision laid down u/s.44AD and if he justify his income below 8% of his turnover then he has to maintain the books of accounts and get it audited.
Let me explain through an example for the above situation.
Mr.X is having Rs.21 Lakh turnover. As per Sec.44AD his income will be considered as Rs.168000/- (i.e. 8% of Rs.21 Lakh). however, if Mr.X is claiming that his income is below Rs.168000/- then he has to maintain necessary books of accounts and get it audited.